Our laboratory is pursuing a multi-faceted research projects to understand the molecular mechanism of the enzyme 3-hydroxyl-3- methylglutaryl co-enzyme A (HMG-CoA) reductase by a combination of biophysical and X-ray crystallographic methods. From the 2.8A structure of the Pseudomonas mevalonii enzyme and a number of binary and ternary complexes with substrates, we have outlined a catalytic mechanism which we believe applies to all members of this class of enzymes. We now propose to investigate the mechanistic proposal with a combination of crystallographic and kinetic measures, with the goal of trapping structurally significant intermediates in the catalytic reaction, and will extend these studies to the HMG-CoA reductases of a number of different species. We will also begin to investigate the structural basis of the modulation of activity in HMG-CoA reductase by reversible phosphorylation in the mammalian enzymes, while attempting to crystallize one of the mammalian reductases. Finally, we intend to begin investigations of species specific differences in the active sites of HMG-CoA reductases from bacteria, archeabacter and eukaryotes which may lead to the development of species-selective inhibitors for this enzyme. The specific aims of this proposal are to: 1) Complete the crystallographic studies of the enzyme-substrate complexes involved in the molecular mechanism of Pseudomonas mevalonii HMG-CoA reductase. 2) Use steady-state and fast reaction kinetics with possible suicide inhibitors to identify and characterize intermediates produced during the Pseudomonas HMG-CoA reductase reaction which can be examined by X- ray crystallographic techniques, and to extend these studies to the Sulfolobus and Syrian hamster enzymes. 3) Investigate the basis of phosphorylation control of HMG-CoA reductase with X-ray crystallographic studies of an engineered Pseudomonas enzyme. 4) Express, purify, characterize and crystallize representative HMG-CoA reductases from the Class I and Class II enzymes in order to examine how the observed differences in their biological and biochemical properties are expressed in their structures. 5) Use the crystallographic structures for these enzymes to identify differences that could be exploited to design species/specific inhibitors for HMG-CoA reductase.